The present invention relates to a coding system for identifying reference marks in a measuring instrument of the type which includes a measuring scale that defines a graduation and a plurality of identical reference marks positioned alongside the graduation, a scanning unit comprising means for scanning both the graduation and the reference marks, and means for utilizing control pulses generated from the reference marks.
In such a measuring system the electrical control pulses generated in response to the reference marks can be used in a variety of ways. For example, such control pulses can be used to set a counter included in the measuring system to the value zero, to load a predetermined measurement value into the counter at the beginning of a measuring operation, or to control interference pulses. In addition, such control pulses can be used to control a control arrangement engaged on the outlet side of the measuring system.
German Patent DE-PS No. 24 16 212 discloses a prior art incremental length or angle measuring system which includes an incremental graduation and a series of reference marks provided on a separate track alongside the graduation. The absolute values of these reference marks are determined from the differing spacings between individual ones of the reference marks. The spacings between the reference marks are determined by scanning the incremental graduation. With this system in order to determine the absolute position of any single reference mark, at least two reference marks must be scanned. This process is relatively complicated and time consuming if for example two such reference marks lie far apart. Moreover, in the event the increments between two reference marks are counted erroneously, an erroneous spacing can be measured and thereby erroneous absolute values can be assigned to reference marks.
German Patent DE-PS No. 29 52 106 describes an incremental length or angle measuring system in which a plurality of reference marks are provided on a scale alongside the measuring graduation. In the disclosed system each of the reference marks is provided with a unique line group distribution, different from that of the other reference marks. The individual reference marks are scanned by separate scanning fields included in the scanning unit, each scanning field being clearly allocated to a selected one of the reference marks having a corresponding line distribution. This approach is relatively expensive to implement, since the line group distributions of the individual reference marks should differ as much as possible one to another in order to permit an unambiguous identification of the individual reference marks. The scanning unit must include a separate corresponding scanning field for each of the reference marks to be identified.
German DE-OS No. 30 39 483 discloses an incremental length or angle measuring system in which a reference mark track and a code track are arranged parallel to one another alongside a graduation track. In this system separate scanning fields in the scanning unit are required for the scanning of the reference marks and the associated code marks. Each portion of the code mark is allocated a respective scanning field on the scanning plate.